The lighter airplane is flying too fast, too low, and too nose-down.
There is an immensely complicated interrelation of variables that determines an aircraft's aerodynamic efficiency in Steady Level Unaccelerated Flight (SLUF), or cruise, but some of them are required lift (Which, in cruise, equals weight), airspeed, altitude, and (It's not usually looked at this way, as it is more often a function of coefficient of lift, but it can be derived at a particular airspeed and altitude) angle of attack.
By the constraints of the table, you have locked in altitude and, to some extent, airspeed (Because dispatchers don't like to slow the airplane down during the trip). The remaining variable to take lift away from the aircraft (When it is lighter) is angle of attack. Unfortunately for this example, there is an angle of attack for optimal aerodynamic efficiency given values for all the other variables, and on either side of that angle of attack, the lift-to-drag ratio falls off.
Bluntly, you are accepting an aerodynamic penalty for operating at a preferred altitude and airspeed. An aircraft which is operating light of optimal can get closer to an optimal angle of attack by either climbing or slowing down, either of which will comparatively raise the nose once SLUF is reestablished.
You, as an aspiring dispatcher, know why they don't like to slow down.
Most long-range aircraft will do some amount of climbing after the initial cruise segment as they get lighter to regain efficiency, but ATC and operational considerations, aircraft limitations, weather, and cost/benefit for the remaining sector length can lead to suboptimal aircraft operation.
So, the engineering answer, which may be more complicated than they're looking for or something entirely different than what they're looking for, is that the lighter aircraft is flying too low and too fast for optimal aerodynamic efficiency (As this results in an attitude nose-down of optimal).